The present invention relates to image forming apparatuses such as a digital copying machine and a laser printer, in which a photosensitive drum is scanned and exposed by laser light beams to thereby form an electrostatic latent image on the photosensitive drum, particularly to a laser control circuit which controls a light emitting timing of laser.
As an image density reproduction system applied to an electrophotographic image forming apparatus using light beams, there has heretofore been a pulse width modulation system (PWM). In the pulse width modulation system, an image signal of each pixel of document image data is converted into a pulse width signal, and laser of an exposing unit is modulated by the pulse width signal. For example, when an image signal is FFh, a light beam is applied to the whole surface of one pixel on a photosensitive member. When the signal is 80 h, the light beam is applied to a region of a half of one pixel.
In the PWM, PWM circuits are provided for an odd-numbered pixel (odd pixel) and an even-numbered pixel (even pixel) of the image data, respectively. A PWM output timing is shifted by a half period between the odd and even pixels. Accordingly, the pixel can be output at an image clock rate which is twice a maximum rated operation speed of the PWM circuit. At this time, each pixel value of the document image data is set to a half value beforehand, and then input into each PWM. In a system in which two PWM circuits are provided in this manner, when the maximum rated operation speed (maximum input clock rate) of each PWM circuit is, for example, 100 MHz, a modulated pixel can be output at 200 MHz.
In this system, to enhance an image quality, an output pulse of each PWM circuit needs to be partially masked (deleted). A masking period corresponds to a latter half period of a clock input into each PWM circuit.
In a system in which the output pulse of the PWM circuit is partially masked, delay of a mask generation timing and fluctuation of a mask width are caused by influences of an operation delay time of a logic circuit for producing a mask, ambient temperature and the like. Therefore, a width of pixel is smaller than a width to be originally output, or conversely broadens. When precision of the pixel width drops, a quality of a printed image drops.